The invention relates to the field of x-ray and gamma-ray analysis, particularly, but not only, high flow.
The applications of x- or γ-rays have developed in the field of non-destructive testing and in security applications (detection of explosive materials using multi-energy radiography, for example).
One particular industrial application of the invention is the detection of explosives to check luggage using continuous radiography. But other applications are also possible, in particular during intense X and/or gamma photon flow measurements by measuring the photon flow transmitted by the sample.
Moreover, the known techniques are very difficult to make compatible with the current requirements for luggage inspections: the method must be fast, but also precise and compatible with security. In particular, the conveyance speed of the luggage requires that the energy of the photons transmitted through the luggage be measured, generally over a short time (several ms) with an incident photon flow that may be high (several tens of Mphotons/mm2/s) to keep sufficient statistics.
To that end, various types of detectors may be used, including sensors measuring an average deposited energy or, more recently, spectrometric sensors. In this type of application, the object to be tested is placed between an ionizing ray source, generally an x-ray source, and the detectors. As a result, the detectors measure the radiation transmitted by the object.
In the prior art, the latter parts are generally non-spectrometric detectors, delivering a signal depending on the intensity of the X radiation. These are for example scintillation detectors not having a spectrometric function. Such detectors are superimposed on one another, intermediate screens being able to be placed between two successive detectors. Generally, two detectors are used, under the name of “sandwich sensors”: a first detector superimposed on a second detector, the first detector being placed near the object to be tested. The first detector is generally small, such that it primarily absorbs the low-energy photons. The second detector is generally larger, so that it primarily absorbs the high-energy photons. Thus, by using these first and second detectors, one respectively measures an intensity of the low-energy component and an intensity of the high-energy component of the radiation transmitted by the object. By comparing these measurements to measurements done with the same detectors, but without the object between these detectors and the source (or direct measurements), attenuation coefficients of the object are estimated, at low energy (using the first detector) and high energy (using the second detector).
The measured attenuation coefficients are then compared with the reference coefficients obtained in the same way, the object then being replaced by reference materials, with known thicknesses and natures.
Finally, this amounts to determining which of the reference materials provides reference attenuation coefficients closest to those measured with the analyzed object. It is then considered that the material of the analyzed object has the characteristics (nature, thickness) of this reference material said to be closest.
Recently, superimposed detectors have advantageously been replaced by a detector having a spectrometric function. It is then possible to obtain a transmission function of the object subjected to X radiation. From this function, it is possible to determine the attenuation coefficient parameters, in different energy ranges, which can be compared to the parameters of known materials.
Thus, irrespective of the detection technique used (superimposed non-spectrometric detectors or spectrometric detector), the problem arises of identifying a material by comparing attenuation coefficients measured on an unknown object to reference coefficients done on reference materials.
The known approaches are based on identifying the reference material having the attenuation coefficients closest to those established with an unknown object. But they are not reliable, in particular when the measurements are done quickly. The signal acquisitions being short, the related uncertainty is high. The problem also arises of finding a method for identifying a material, using x or γ rays, which is more reliable than the methods currently known.
Lastly, as already explained, one of the applications is luggage inspection, for example in an airport, for explosives detection.
Now, in this case, the problem arises of performing an inspection very quickly, so as to be able to examine luggage successively, in very short periods, compatible with the arrival of travelers' luggage in a detection device.